The present invention relates to infrared detectors used primarily for detecting gas concentrations in a confined space, usually defined as a fixed distance of space between an infrared source and an infrared detector. More particularly, the invention relates to electronic control circuits for receiving analog electrical signals from such infrared detectors, and automatically compensating for variables in signal and circuit parameters.
The measurement of gas concentrations by using infrared detectors is typically accomplished by application of Beer""s Law, which can predict the amount of absorption of an infrared signal wavelength passing through a region of concentration of a particular target gas, according to the formula:
I=Io exe2x88x92xcex3lc; where
I is the signal after absorption;
Io is the signal before absorption;
xcex3is the absorption coefficient of the gas;
l is the path length through the gas; and
c is the gas concentration.
In designing such a measurement system, it is common to use two infrared channels, each developing an infrared signal at a different frequency (wavelength), where one wavelength is known to be particularly sensitive to absorption in the chosen target gas, and the other wavelength is known not to be particularly sensitive to absorption in the chosen target gas. Therefore, the concentration of the target gas can be measured by taking the ratio of the signal strength of the non-sensitive wavelength (the reference channel) to the signal strength of the sensitive wavelength (the analytical channel).
The two infrared channels can be developed using a single infrared source positioned at a fixed distance from two wavelength bandpass filters which are sensitive to various wavelengths as described above, and permitting the target gas to fill the space and distance between the source and the two filters. One channel is chosen so as to pass frequencies in the infrared band, but the bandpass filters are centered on a wavelength known to be highly absorbed in the target gas; this channel is known as the xe2x80x9canalyticalxe2x80x9d channel. The other channel is chosen so as to pass frequencies in the infrared band, close to the bandpass frequencies of the first channel, with the bandpass filters centered on a wavelength which is not highly absorbed in the target gas; this channel is known as the xe2x80x9creferencexe2x80x9d channel. For example, if measuring a hydrocarbon gas such as methane, the reference channel sensitivity would be set to a wavelength of 3.9 microns, and the analytical channel sensitivity would be set to a wavelength of 3.4 microns. Most of the environmental factors which may be present will affect both the analytical and the reference channel in a similar manner, but the presence of a target gas affects primarily the analytical channel. A quotient, formed by dividing the reference signal value by the analytical signal value, will only change when a target gas is present in the optical path of the infrared channels; the amount of quotient change can be used to calculate the target gas concentration using Beer""s Law.
There are a number of factors which can degrade accuracy in the foregoing scheme. The infrared sensors are necessarily analog devices, although the time-varying analog voltage signals they produce are frequently converted to digital values through the use of analog-to-digital converters (A/D converters) to obtain greater precision. If the analog voltage signals become too large they can exceed the bounds of the A/D converter; if the analog voltage signals become too small they produce poor resolution of the digital converted value, and both of these factors can lead to a reduction in accuracy of the measured values. Another cause of reduced accuracy is component tolerance variations, particularly in the photo elements, which make it difficult to control analog voltage levels from unit to unit; this can be addressed by providing adjustable potentiometers in the circuits, for making tolerance adjustments during the manufacturing process. Another cause of reduced accuracy, after units are placed into operation, is that the optical components of a system can become coated with dust or other substances which attenuate the light passing through, reducing the analog voltage signal levels produced by the optical detectors; similarly, the output light intensity level typically lowers as a light source ages, which also reduces the analog voltage signal level. These problems can been addressed by frequently cleaning the optical path components and using manual potentiometer adjustments as the equipment ages.
Another cause of inaccuracy is the variation in absorption coefficients of different gases, which causes the absorption response curve to vary from gas to gas, and also from one gas concentration level to another. This problem can be addressed by choosing specific, selectable, families of response curves for specific gases, and different analog voltage gain values for the electronic circuits used to detect the gas concentrations. Another cause of inaccuracy is temperature variations in the measurement environment, which can cause changes in circuit performance in the measurement system.
Most of the foregoing problems are more or less continuously present, and frequent readjustments of circuit parameters and frequent cleaning are inadequate for ensuring a smooth, continuously accurate measurement result. It is therefore a principal object of the present invention to provide a stable, continuously accurate measurement system for monitoring gas concentrations in a particular environment. Specifically, it is an object of the present invention to provide a circuit for automatically adjusting analog voltage signal levels to keep the permissible signal range constant, ie., to maintain the maximum permissible signal at a constant voltage span relative to the xe2x80x9czeroxe2x80x9d signal level; it is also an object of the invention to provide circuit compensation for variations or drift in the xe2x80x9czeroxe2x80x9d or reference voltage signal level, and for circuit gain variations caused by any of the aforementioned effects. The foregoing objects advantageously provide a circuit for automatically controlling variables related to measuring analog voltage signals from infrared gas detection devices, by providing a constant zero point for analog voltage signals, a constant voltage value representing the full-scale point for the same signals, an adjustable gain circuit to compensate for changes in overall circuit gain, and an absorption coefficient response curve for any known gas, which is accurate to a first order approximation.
A control and measurement circuit responsive to analog voltage signals produced by infrared sensors in a gaseous environment, including a computer processor connected to receive signals from the infrared sensors after the circuit""s zero reference voltage has been calibrated by a BALANCE circuit, the circuit""s gain has been adjusted by an AGC circuit and the circuit""s voltage range has been set by a SPAN circuit, wherein each of these circuits has a digitally-controllable potentiometer as an input impedance, wherein each potentiometer is adjustable by binary voltage feedback signals generated by a computer processor. As a result, the analog voltages present within the circuit during the calibration process are converted to digital values which are used by software within the computer processor to generate digital feedback signals to the respective BALANCE, AGC, and SPAN circuits to adjust the respective circuit""s analog voltage response characteristics.
The computer processor receives the analog voltage signals generated by the infrared sensors and converts them to digital values for internally comparing the digital values to a prestored Beer""s Law curve for the particular gas being measured, for providing a measurement of the concentration of the gas being measured.